Electrical control circuit



Feb. 18, 1941. A. H. MYLES 2,232,257

ELECTRICAL CONTROL CIRCUIT Original Filed July 16, 1938 H 12 vlw 5 I35:'4 ll L H5;

FIGJl Ll, L2 L3,

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I CAPACITIVE A I INDUCTWE E (I S 01 D o 5 5 INVENTOR.

FREQUENCY CYCLES/SEC. ASA .M I ES FIG-3 HIS ATTORNEY.

Patented Feb. 18, 1941 UNITED STATES PATENT OFF-ICE ELECTRICAL CONTROLCIRCUIT Asa H. Myles, Euclid,

hio, assignor to The Electric Controller & Manufacturing Company, iCleveland, Ohio, a corporation of Ohio 9 Claims.

This invention relates to electrical control circuit-s, and particularlyto control circuits which depend upon the phenomena of resonance fortheir operation.

This application is a. division of my co-pending application filed July16, 1938, Serial No. 219,585, which describes and claims an electricalcontrol system and method for synchronizing a pair of synchronous-tiemotors while they are rotating, and in which system and method thepresent in vention is utilized. The invention is shown and describedherein for illustrative purposes only as applied to the control of theacceleration of a wound rotor induction motor and is claimed broadly foruse in a control circuit which may have other uses than that shown byway of illustration. Embodiments of the invention in connection" withother electrical devices are apparent from the illustrated example andwill not be specifically described.

Under normal operating conditions, the flux in the air gap of apolyphase induction motor is constant, and the magnitude and frequencyof the induced rotor voltage are each dependent upon the difference inspeed between the rotating magnetic field set up by excitation of theprimary windings and the speed of the rotor in space. The magnitude ofthe induced rotor voltage at standstill is equal to the magnitude of thesupply voltage, if it is assumed that the motor has a transformationratio of unity, and the frequency of the induced rotor voltage atstandstill is equal to the frequency of the source of supply.

During acceleration of the motor, both the frequency and magnitude ofthe rotor voltage decrease in accordance with the increase in speed ofthe rotor so that, at normal operating speeds, the induced rotor voltagebecomes extremely small and of low frequency, and at synchronous speedthe rotor voltage is zero. If the motor is driven above synchronousspeed, the magnitude Since both the magnitude and frequency of the rotorvoltage vary concurrently in the same direction with respect to eachother, an ordinary inductive relay cannot be made responsive to thevariations. The reason for this is that asthe frequency declines, thereis a tendency or more relay current to flow through the inductivecircuit, but, since the voltage is concurrently declining, the relaycurrent remains substantially constant. The same result occurs if thefrequency and voltage are concurrently increasing.

In a co-pending application of John D. Leitch, which resulted in PatentNo. 2,165,491 on July 11, 1939,, a series resonant control circuitincluding a relay coil is shown connected for energization to the rotorcircuit of an induction motor. At standstill the current through therelay coil is sufficient to cause operation. During acceleration of themotor, because of the phenomenon of series resonance, the currentthrough the relay coil suddenly decreases at a predetermined frequencyand magnitude of rotor voltage, and the relay operates to perform acontrol function upon the motor.

The present invention utilizes the phenomenon of parallel resonance inaddition to the phenomenon of series resonance. As a result of theconnections giving the additional resonant condition, a sharper changein the control circuit current occurs at the predetermined electricalcondition, and higher currents may be caused to flow through the controlcircuit under electrical conditions in which the voltage and frequencyare higher than the resonant values. Thus the addition of the parallelconnected inductance to the series-resonant circuit increases the degreeof the control circuit current change as the control circuit passes froma capacitive to an inductive condition.

The improved control circuit of this invention includes a non-saturableinductance and a condenser connected in series with each other and anadditional non-saturable inductance connected in parallel with thecondenser. The additional inductance and the condenser are adapted tobecome parallel resonant at a predetermined electrical condition, andthe condenser and the series connected inductance are adapted to becomeseries resonant at an electrical condition almost the same as theparallel resonant condition. At parallel resonance the impedance of theparallel connected condenser-inductance combination is a maximum, andthe total current through the control circuit is accordingly small. Atseries resonance the impedance of the series connectedcondenser-inductance combination is small, and the total current throughthe control circuit is a maximum.

It was found experimentally that under electrical conditions in whichboth the frequency and voltage are higher than the resonant values, morecurrent flows through the control circuit than would normally beexpected, and under electrical conditions in which both the frequencyand voltage are lower than the resonant values, an extremely smallcontrol circuit current flows. For this reason, if the series connectedinductance is a relay coil, as in the illustrative example, the relaycurrent when the relay is connected to a rotor circuit of an inductionmotor is lar e at standstill, and small after the resonant conditionshave occurred. Electrical responsive means other than a relay ifassociated with the circuit would correspondingly be positively actuatedby such definite and great changes in current flow- One of the principalobjects of the present invention is to provide a new and improved meansresponsive to the electrical condition of a circuit in which thefrequency and voltage are varying concurrently in the same direction.

A correlative object is to provide a new and improved means responsiveto the electrical condition of a secondary circuit of an alternatingcurrent motor for controlling the operation of the motorin accordancewith said condition.

Another object of the invention is to provide, in combination with aresonant control circuit, a means for altering the normal operativeeffect resulting from a change from a predetermined condition to adifferent condition, one of which conditions is series resonant and theother of which conditions is not series resonant, the circuit includinga capacitance and an inductance which are connected in series and areresponsive to coexistent variations in voltage and frequency of a supplycircuit, and which become series resonant at a predetermined value ofsaid voltage and frequency when'said voltage and frequency are varyingconcurrently in' the same direction.

Another object of the invention is to provide a control circuit which,when connected for energization to a power circuit in which the voltageand frequency are varying concurrently in the same direction, becomesseries resonant under some electrical conditions of said power circuitand parallel resonant under other electrical conditions.

A further object of the invention is to provide a doubly resonantcontrol circuit responsive to the electrical condition of a secondarycircuit of an alternating current motor for controlling the operation ofthe motor in accordance with said condition.

A further object of the invention is to provide a resonant controlcircuit which, when connected for energization to a secondary circuit ofan alternating current motor, is extremely sensitive to variations inthe electrical condition of the secondary circuit.

A ..ore specific object is to provide a relay circuit including aninductance and condenser connected in parallel and in series with arelay coil, the inductance and condenser combination being adapted tobecome parallel resonant when a predetermined voltage of a predeterminedfrequency is impressed thereon, and the relay coil and condenser adaptedto become series resonant when a different predetermined and in whichthe control circuit current is much greater at low speeds than at higherspeeds.

Other objects and advantages will become apparent from the followingspecification, wherein reference is made to, the drawing, in which Fig.l is a simplified wiring diagram illustrating the invention;

Fig. 2 is a simplified wiring diagram illustrating the invention and themanner in which the invention may be used for controlling the operationof a wound rotor induction motor; and

Fig. 3 is a graph showing comparative frequency and voltage responsecurves between the present invention and the prior art.

In Fig. 1, 10 represents a source of electrical energy having a variableelectrical condition in which the voltage and frequency concurrentlyvary in the same direction. This interrelation of voltage and frequencyoccurs in the output from a synchronous generator as it comes up tospeed, and also is present in the secondary circuits of polyphasealternating current motors during acceleration or deceleration. Anonsaturable inductance H, a condenser I2, and an additional inductanceillustrated as an operating winding I3w for a relay 13 are connected ina control operating circuit H across the source It. The inductance IIand condenser l2 are connected in parallel with each other and in serieswith the winding liw. The relay II has contacts In which may controlsome electrical device in accordance with the electrical condition ofthe source I I.

Referring to Fig. 3, curve A is a typical current-frequency responsecurve for a series resonant circuit under a variable electricalcondition in which the voltage and frequency vary concurrently in thesame direction. It will be noted that curve A on the capacitive side ofthe resonance point m slopes gradually over a wide range of frequenciesto a point n which may be considered the deenergization point. It istrue that by lowering the resistance of such a control circuit, theslope of this portion of the curve could be made much steeper, but froma practical standpoint of an operative control circuit design, this isimpossible. It will also be noted that over the portion 0 of the curve Ain the higher frequency ranges, the control circuit current is not muchgreater than it is at the point 11. It is obvious from the curve A thatthe current at the point n must be as high as possible to obtain adefinite cut-off and also that the current at point 1: cannot be higherthan the current at point 0. Therefore, if a relay is used, the currentin the relay circuit at pick-up cannot be much greater than at drop-out,and a tendency toward erratic operation under some conditions results.

A series resonant circuit gives minimum impedance at resonance for afixed total resistance, and, as is well known, parallel resonance givesmaximum impedance at resonance for a fixed total inductance. Thisdifference between the impedance of a series circuit and of a parallelcircuit at resonance is of great importance in radio, as by its use itis possible to tune a radio receiver so that it will respond to adefinite frequency and at the same time suppress an undesirablefrequency.

Because of the inherent high resistance of the operating winding of anyelectro-responsive device such as a relay sensitive enough to respond tovariations in the electrical condition of the secondary windings ofalternating current motors, the drop in the current-frequency curve onthe capacitive side of resonance in a series resonant circuit cannot bemade steep enough for all purposes in the region of current valuesbelow'the current values on the inductive side which must causeenergization of the relay. A series-parallel resonant circuit tuned togive series resonance at, for instance, 30 cycles, and parallelresonance at 28 or 29 cycles, was found to give a steeper drop incurrent in the desired region than had heretofore been obtainable.Furthermore, it was found that the current on the inductive side of theresonance point remained at an extremely high value, thus eliminatingthe erratic operation which sometimes occurred when a series resonantcircuit alone was used. This advantageous result can be explained by thefact that the parallel combination at voltages and frequencies aboveresonance is capacitive and has a greater'oapacitive eflect than if thecondenser alone were present, resulting in a lower total impedance atall frequencies and voltages above resonance.

One way in which the control circuit M of Fig. 1 can be used to controlan induction motor during acceleration is shown in Fig. 2. It isunderstood that several steps of acceleration can be provided by merelyusing extra relay circuits and contactors in the same manner as shownfor the single step, the respective relay circuits being adjustable tobecome resonant at difierent elec-' trical conditions. Such a controlsystem using series type resonant circuits is disclosed in theaforementioned Leitch application. Furthermore, series resonant relaycircuits and seriesparallel resonant relay circuits could be combined inone controller ii such were desired.

In Fig. 2 a motor 20 is shown as a three-phase wound rotor inductionmotor provided with Y- connected secondary resistors 2! which may beshort circuited by means of an electromagnetic contactor 22 operable bya winding Etc). The energizing circuit for the winding 2210 is completedthrough the normally closed contacts to of the relay 13. The operatingcoil ifiw for the relay i3 is part of the control circuit it which isthe same as that shown in Fig. 1 and like parts thereof are referred toby like numerals. The control circuit H is energized from the secondarycircuit of the .motor 20 throughan auto transformer l5. Other means forconnecting the control circuit N to the secondary circuit may beemployed, such as, for example, a direct connection across two terminalsof the secondary winding or across a portion of the accelerationresistor.

The primary winding of the motor 20 is arranged to be connected to thesource of power indicated by the conductors L1, L2 and In by means of anelectromagnetic contactor 25 having an operating winding 25w andnormally open auxiliary contacts 25a. A switch 26 is arranged to controlenergization of the winding 25w and consequent operation of thecontactor 25.

in operation, closure of the switch 25 cornpletes a circuit through thewinding 25w from the conductors L2 and L3. The contactor 25 response toenergization of its operating winding 25w closes its main contacts toconnect the conductors L1, L2 and L3 to the primary winding of the motor20. Immediately thereafter there is induced in the secondary circuit ofthe motor 20 a voltage of considerable magnitude and having a frequencyequal to line frequency. This voltage is impressed through theauto-transformer it across the control circuit H and a currentcorresponding to portion 8 of curve B of Fig. 3 is caused to iiowthrough the winding 13w of the relay I3. The relay I3 in response toenergizetion of its operating winding Ilw by a current such as indicatedby portion s of curve B opensits contacts Na to interrupt the circuit tothe operating winding 22w oi the accelerating contaotor M. Theuncompleted circuit to the operating winding 22w is irom the conductorL2, the now closed contacts 25a, the now open contacts 33a, and thewinding 22w, to the conductor L3.

As the motor 26 accelerates due to the connection of its primary windingto a source of alternating current, the magnitude and frequency of theinduced rotor voltage gradually decrease. Due to the phenomenon ofseries resonance, however, the current through the winding i-llwgradually increases at this time. The capacity of the condenser i2 is sochosen in relation to the in-.- ductance of the magnetic circuit of therelay 63 that a series resonant condition occurs at some predeterminedspeed of the motor 20, such as the speed which will cause the inducedvoltage to have a frequency of thirty cycles. The additional inductance,by increasing the capacitive reactance of the circuit, causes the seriesresonant condition to occur when the voltage and re uency are slightlyhigher than when the inductance is not present. impedance of the seriescircuit including the condenser l2 and the relay winding i311) causes agradual rise in relay current until the point r of curve B is reached.By virtue of this, increased relay current, the relay contacts 83a arepcsia tively held in the open position during this ne riod. The additionof the inductance ii in parailel with the condenser l2 results in ahigher relay current duringthis interval as indicated by the differencebetween the portion 0 of curve and the portion 5 of curve B.

Sshortly after the frequency and voltage reach the predetermined values,the series-resonant condition disappears, and the control circuit current starts to fall. At a slightly lesser value of frequency and voltagethe parallel circuit in cluding the condenser iii and the inductance iiis designed to become parallel resonant, resuiting in a greatlyincreased impedance. its a resuit the relay current drops to nearly zeroalmost instantly. The winding 43w is therefore pram tically deenergizedand the relay iii thereupon closes its contacts If-ta to permitenergization of the winding The contactor 22 in response to energiaationof its operating winding 222w closes its contacts to short circuit theresistor iii to permit the motor to accelerate to normal speed.

Deenergization of the relay i3 causes a change in its magnetic circuitand thus changes the inductance of the operatingwinding iliw. change ininductance is such that the series res onant condition cannot occuragain unless a somewhat higher value of frequency and voltage than theresonant frequency and voltage is impressed on the circuit it. Thisfundamental iact prevents fluttering of the relay during closing.

To stop the system, it is only necessary to open .the switch 28, whichwill interrupt the circuit to The gradual reduction in nected in seriesand are responsive to coexistent variations in voltage and frequency,when said voltage and frequency are varying concurrently in the samedirection, to change from a predetermined condition to a diiferentcondition and which are in series resonant condition at a predeterminedvalue of said voltage and frequency, one of said conditions being seriesresonant and the other of said conditions not being series resonant, andan additional non-saturable inductance connected so as to form aparallel circuit with the capacitance and so related in electrical valueto said capacitance as to be operative to render the said parallelcircuit resonant at a different predetermined value of said voltage andfrequency for altering the normal operative effect of said change.

2. A control circuit comprising a capacitance and a non-saturableinductance which are connected in series and are responsive .tocoexistent variations in voltage and frequency to become series resonantat a predetermined value of said voltage and frequency when said voltageand frequency are varying concurrently in the same direction, and tochange from a series resonant condition to a condition which is notseries resonant, and an additional non-saturable inductance connected soas to form a parallel circuit with the capacitance and so related inelectrical value to said capacitance as to be operative to render thesaid parallel circuit resonant at a diiferent predetermined value ofsaid voltage and frequency for altering the normal operative eilect ofsaid change.

3. A control circuit comprising a capacitance and a non-saturableinductance which are connected in series and are responsive tocoexistent variations in voltage and frequency to become series resonantat a predetermined value of said voltage and frequency and to changefrom a condition which is not series resonant to a series resonantcondition, when said voltage and frequency are varying concurrently inthe same direction, and an additional non-saturable inductance connectedso as to form a parallel circuit with the capacitance and so related inelectrical value to said capacitance as to be operative to render thesaid parallel circuit resonant at a different predetermined value ofsaid voltage and frequency for altering the normal operative effect ofsaid change.

4. A control circuit comprising a capacitance and a non-saturableinductance which are connected in series and are responsive tocoexistent variations in voltage and frequency, when said voltage andfrequency are varying concurrently in the same direction, to change fromone condition to another, one of said conditions being inductivelyreactive and the other of said conditions being capacltively reactive,and an additional non-saturable inductance connected so as to form aparallel circuit with the capacitance and so related in electrical valueto said capacitance as to be operative to render the parallel portion ofthe control circuit resonant at a predetermined value of said voltageand frequency for altering the normal operative efl'ect of said change.

5. A control circuit including a non-saturable electromagnetic relayoperating means and relay operated thereby for controlling a powercircuit, a capacitance, the capacitance and the relay operating meansbeing connected in series and responsive to coexistent variations involtage and frequency to change from a condition which is notserlesresonant to a series resonant condition, when said voltage and frequencyare varying concurrently in the same direction, and operatlve upon saidchange to operate the relay, and capable of becoming series resonant ata predetermined value of said voltage and frequency, and a non-saturableinductance connected so as to form a parallel circuit with thecapacitance and so related in electrical value to said capacitance as tobe operative to render the said parallel circuit resonant at a diiferentpredetermined value of said voltage and frequency for altering thenormal operative eflect of said change.

6. The combination with a circuit providingsubjected to a high currentat relatively high frequency and large voltages, a higher current atintermediate frequencies and voltages, and" substantially no current atfrequencies and voltages lower than said intermediate frequencies andvoltages. a

8. A relay circuit adapted for operation by asource of. electrical powerhaving a decreasing voltage and a declining frequency and including anon-saturable inductive operating winding for the relay, a non-saturableinductive circuit, a

capacitive circuit, said inductive circuit and said capacitive circuitbeing connected in parallel cir-' cult relation with each other, meansto connect the parallel circuit including the inductive circuit and thecapacitive circuit in series with the op' erating winding across thesource of power, the relative values of the inductance of the inductivecircuit, the capacitance of the capacitive circuit,

and the inductance of the operating winding being such that at apredetermined value of frequency and voltage the current in theoperating winding declines from a maximum to a minimum and, at allhigher values of frequency and voltage, is substantially above saidminimum.

9. A resonant relay circuit comprising 9. non satura-ble electromagneticrelay operating means,

a condenser, said means and said condenser being connected in series andresponsive to coexistent variations in frequency and voltage so as tobecome sen'es resonant at a predetermined value of said voltage andfrequency, and a non-saturable inductance connected in parallel withsaid condenser and being so related to the reactance of the condenserand the relay operating means that a greater variation exists betweenthe ca pacitive reactance and the inductive reactence of the relaycircuit as the relay circuit is caused to pass thru series resonance dueto variations in said voltage and frequency.

ASA H. MYLES.

